NO752592L - - Google Patents

Description

The present invention relates to a method for the production of chlorinated ethylene derivatives, whereby chlorine-containing by-products are burned in a catalytic combustion reactor with a fluidized bed, whereby hydrogen halide is mainly produced, which is recycled to the reactor for the production of chlorinated derivatives, and the heat of combustion from the reactor is used for preheating the compounds introduced into the reactor. The catalyst in the fluidized layer contains SiOp and has a surface area of at least 10 m/g. The catalytic combustion reaction with a fluidized bed is carried out under atmospheric pressure or superatmospheric pressure and temperatures in the range 350-55 (D°CJ whereby a mixture of gases is obtained which essentially contains hydrogen chloride and carbon oxides, water and inert compounds, essentially free of elemental chlorine and chlorinated hydrocarbons.

The present invention generally relates to the production of chlorinated ethylene derivatives such as vinyl and vinylidene halides, especially vinyl chloride. Closely related to this is also the synthesis of chlorinated solvents of ethylene, chlorine and/or hydrogen chloride. Among such solvents are the highly chlorinated ethenes such as perchloroethene, which are produced by a method where the ethene and/or partially chlorinated ethanes undergo one or more steps of catalytic oxyhydrochlorination with hydrogen chloride and

• oxygen.

Vinyl chloride is produced according to various processes from ethylene, elemental chlorine and/or hydrogen chloride, whereby in most cases a cracking step is used where ethylene dichloride is thermally cracked in the gas phase under pressure to vinyl chloride and the by-product hydrogen chloride. The latter is recovered through an oxyhydrochlorination step in which the hydrogen chloride is reacted with further ethylene and oxygen to produce dichloroethanes which in turn are recycled to the cracking step. In many processes, a direct chlorination step is also used where ethylene and elemental chlorine are reacted in the liquid phase to produce dichloroethanes which are then cracked into vinyl chloride.

In all these known solvent and monomer processes, the relevant steps comprising direct chlorination, oxyhydrochlorination and/or cracking are not 100% selective for the formation of the desired chlorinated hydrocarbon end products,

and as a result of this, quite large amounts of unwanted chlorine-containing by-products are obtained in the form of complex mixtures which, with varying composition, contain from chloroform or ethyl chloride to trichloroethanes and trichloroethenes, tetrachloroethanes, hexachloroethanes, hexachlorobutadiene etc., as well as aromatic compounds. These unwanted chlorine-containing by-products obviously cause economic and ecological waste problems.

It would therefore be desirable to have a process for the production of chlorinated derivatives of ethylene and chlorinated solvents of ethylene, chlorine and/or hydrogen chloride where the generated heat energy is utilized in the process and the production of unwanted chlorinated by-products of hydrocarbons is essentially eliminated or not is expressed in the end result due to the reuse of the waste products in the process.

It has now surprisingly turned out that the above-mentioned problems with the previously known processes can be avoided or substantially remedied by the present method whereby the unwanted chlorinated hydrocarbons as by-products are extracted for recirculation in the form of hydrogen chloride which is essentially free from elemental chlorine and chlorinated hydrocarbons, and which is recycled to the process for the production of chlorinated ethylene derivatives. In addition, the intrinsic heat of the raw by-products is recovered by preheating the starting materials and intermediate products in the process. In particular, the method according to the present invention comprises that the unwanted hydrocarbons containing by-products are passed through a heated layer of silicon dioxide catalyst, which layer is fluidized with air through which said waste products (by-products) are transferred to a stream of combustion gases essentially containing only carbon oxides, water, inert gases and hydrogen chloride.

According to the invention, the terms "chlorinated ethylene derivatives" and "ethylene chloride synthesis" denote the various processes and manufactured products whereby ethylene is reacted with elemental chlorine and/or hydrogen chloride in one or more steps to produce a compound of the type chloroethene or chloroethane, such as vinyl chloride, vinylidene chloride, ethyl chloride, 1,1-dichloroethane, 1,2-dichloroethane, trichloroethanes, trichloroethenes, tetrachloroethanes, perchloroethane and many others. Accordingly, the ethylene chloride synthesis includes any of the steps direct chlorination of ethylene or ethylene chloride derivative, oxyhydrochlorination of ethylene or ethylene chloride derivatives whereby ethylene or a chlorinated ethylene derivative is transferred to products with a higher chlorine content, as well as cracking (dehydrochlorination) or re-claying of the chlorinated ethylene derivatives for the production of those with a higher chlorine content.

When using the present invention,

the industrial by-products containing chlorinated hydrocarbons into and through a layer of silicon dioxide catalyst which is fluidized with air and kept at a temperature approx. 4-00-500°C.

In the catalyst layer, the waste materials are incinerated and transferred to a stream of combustion gases which essentially only contain carbon oxides, water, inert gases and, which is essential, hydrogen chloride. The contact time for the waste materials with the catalyst layer is approx. 10-50 seconds. At the temperatures used, the catalyst causes an essentially complete combustion of chlorinated hydrocarbons in the waste stream, but limits the combustion so that the hydrogen atoms are bound back to the chlorine atoms in the hydrogen chloride. This enables the production of a gas stream which is practically free of elemental chlorine. Elemental chlorine is not desirable and the formation of elemental chlorine must be avoided as much as possible. As complete combustion as possible is also important as the presence of chlorinated hydrocarbons in the combustion gases also tends to increase by-product formation during the oxyhydrochlorination step.

After passing through the catalyst bed, the waste materials are gasified and then burned in the described, regulated manner. Direct injection of the flowing by-product stream, which is often viscous and tar-like and contains substances comprising suspended carbon, does not adversely affect the catalyst bed, nor does it have an adverse effect on the fluidization that takes place in the bed. Feeding of waste substances into the catalyst layer can easily take place using standard equipment such as gear pumps, mechanical displacement pumps and the like. With regard to the temperatures used during the present process, indicated above, there are many known substances that can be used to occupy the catalyst layer and that can withstand the corrosive environment.

The pressure used in the layer of combustion chamber analyzer according to the present invention is not decisive. For example, the catalytic combustion reaction can be carried out

at atmospheric pressure, especially when the combustion gases are not fed directly to the oxyhydrochlorination step or reaction. When the gases are fed in in this way, they must be compressed to the same pressure as prevails in the oxyhydrochlorination reactor, as the oxyhydrochlorination reaction is normally carried out above atmospheric pressure. Consequently, it is desirable to keep the gases in the combustion layer at a pressure in the range approx. 0.17 - 1>05 MPa overpressure and preferably in the range 0.28 - 0.69 MPa overpressure. In most cases, the pressure should be kept exactly slightly higher than the pressure in the oxyhydrochlorination step so that the combustion gases do not need to be pressurized. When carrying out experiments • with regard to testing the present catalytic combustion reaction and using a simulated waste stream, it is of course acceptable and convenient to use atmospheric pressure as there is no need for pressure equipment.

The combustion catalyst on silicon dioxide must have a high surface area, namely at least 10 m^/g. A highly active silicon dioxide catalyst has a surface area in the range of approx. 175-600 m/g. A particularly suitable silicon dioxide catalyst for the present process has a mean diameter approx. 10-100 Ao<,>preferably 20-80 Å. A catalyst with a surface area of approx. 175-550 m /&•

The catalyst components used are common in the trade. When using a fluidized bed, e.g. especially AlgO^ and SiO2 with the random, broad particle size distribution required for good fluidization, namely with some, possibly none, particles smaller than 20 µm or larger than approx. 200 um and where the largest amount of the particles is in the area approx. 4-0 - 14-0 pm. in medium diameter, common in the trade. Very small particles, or the finest grain fractions, which have a mean diameter below approx. 20 µm should be avoided as they are too easily lost from the reactor. Likewise, large particles with a mean diameter greater than approx. 200 pm is avoided as these are too difficult to fluidize. It is obvious that depending on the nature of the present process the catalytic material should not be brittle and should be resistant to the greatest possible extent.

The corrosive action in the catalytic combustion chamber or reactor is very mild in the present process. Common heat exchange coils made of common material and of common design are introduced into the catalyst bed where they serve either as steam-producing tubes or as preheating tubes for raw material or intermediate product supplies to the process for the production of chlorinated ethylene derivatives. Even in such cases when producing chlorinated ethylene derivatives where only approx. 3-8% of the original ethylene input is transferred to by-products, the annual savings in heat energy are considerable. Furthermore, as the present process can operate at low temperatures, combustion gases formed can be fed directly to the oxy-hydrochlorination reaction without intermediate cooling.

As pointed out earlier, the present invention can be carried out with the catalyst in fluidized form and with the use of air as a fluidizing agent or gas. When such a catalyst system is used, the air must be used in sufficient quantity and at a feed rate that not only completely fluidizes the catalyst bed, but is also sufficient for the supply of enough oxygen for the regulated combustion of hydrocarbons in the waste streams (the by-product streams). To guarantee complete combustion of the waste streams, at least 2 mol of oxygen (Og) must be supplied per moles of carbon (Cg) in the waste stream. However, in order to guarantee a sufficient supply of oxygen to the fluidized catalyst bed, a sufficient amount of air is fed in so that approx. 2.5 to 10.0 moles of oxygen are present per moles of carbon in the waste stream. When supply quantities are used that give more than approx. 10.0 mol of oxygen per moles of carbon in the waste stream, this results in reduced capacity and catalyst loss and, which is of greater importance, increases the risk of oxidation of the hydrogen chlorides to elemental chlorine, which must be avoided as previously mentioned. When the air supply rates deliver less than approx. 2.0 mol of oxygen per mol of carbon, only approx. 8O-85% of complete combustion. The best air supply rates are therefore such that approx. 2.5 - 5.5 ml of oxygen are introduced per mole of carbon in the by-product stream.

The contact times between the by-products and the catalyst

in the reactor can vary considerably without the effect of the combustion being particularly affected. When using a fluidized bed reactor, contact times between approx. 5°S 50 seconds, bearing in mind that only approx. half of the calculated contact time represents the time that the gases are really in contact with the layer. This is because. for the rest of the period, the gases are in the free space above the bed in the catalyst release and cyclone separation parts of the reactor. Beneficial results have been achieved with contact times in the range of approx. 15-35 seconds.

As previously pointed out, the most significant variables in the present process are the reaction temperature and the catalyst's effect. When e.g. reaction temperature is lower than 350 C, complete combustion cannot be achieved within acceptable contact times. When the reaction temperature exceeds approx. At 550°C, the combustion reaction mixture is very corrosive and this is of course harmful.

It has been shown that most metal chlorides and metal oxides have a certain catalytic effect when they are used separately in the combustion reaction, but to varying degrees. The difficulty with most of these compounds when used on their own is that they act as Deacon catalysts and consequently transfer or relocate at least part of the amount of chlorine in the waste products to form new polychlorinated hydrocarbons, some of which are more resistant to oxidation.

When one therefore uses such metal catalysts; The combustion gases usually contain considerable amounts of polychlorinated and unsaturated by-products. The catalyst according to the present invention, on the other hand, exhibits the desired catalytic activity and the combustion gases thus produced contain very little and, under optimal conditions, practically no elemental chlorine and essentially no chlorinated hydrocarbons. Furthermore, the catalysts according to the present invention are cheap and robust against contamination with unburnt carbon and the trace metal content from the by-products.

Byproduct streams separated in various fractionation steps in many chlorinated ethylene syntheses contain up to 1 to 2 wt% ferric chloride as an impurity.

In the catalyst layer according to the present invention, iron chlorides and the like are oxidized to finely divided iron oxides and these are transported out of the catalyst layer with the combustion gases and separated in the cyclones. The small amounts of iron oxides that remain in the catalyst layer have no visible harmful effect on the catalyst layer's effect. Nor do small amounts of iron oxides transported out of the combustion reactor and to the following oxyhydrochlorination step seem to affect the oxyhydrochlorination catalyst which is normally located on an aluminum support. The only adverse impact,

if at all, when using pre-combustion gases which are formed according to the present process, in the oxyhydrochlorination step, there is a very small reduction of the capacity due to increased loading of inert gases from the combustion gases in the oxy-hydrochlorination feed.

When using the process according to the present invention in connection with a fluidized bed, the combustion reactor with the solid granulated catalyst is first introduced. After the introduction of air or fluidization, the catalytic layer expands so that it almost fills the reactor's internal volume. The catalyst layer is fluidized in this way before introducing the waste by-product stream. This flow is introduced into the reactor at a level just above the bottom air intake. Preferably, the waste stream is supplied to the reactor through a water-cooled nozzle which prevents evaporation and/or charring of the substances before they come into contact with the catalyst in the bed.

The present invention shall be explained in more detail in the light of the following examples where all quantities and percentages are on a weight basis where nothing else is mentioned.

EXAMPLE I

In this example, three experiments were carried out with varying temperatures and a catalyst of silicon dioxide with a surface area of 150 m^/g which was filled into a reactor with a fluidized bed under atmospheric pressure. Atmospheric pressure was maintained because this makes it easy to carry out the experiment on a small scale. In a factory, however, superatmospheric pressure is preferably used. The amount of catalyst used in each experiment was 180 g. Air was introduced into the reactor in such a quantity that an oxygen/carbon (Og/Cg) ratio of 3.3 : 1»0 was obtained, and the amount of by-product supplied to the reactor was 2 .82 ml/hour. This by-product or waste stream consisted essentially of the following compounds: trans-1,2-dichloroethane, 1,1-dichloroethane, cis-1,2-dichloroethane, CHCl^, ethylene dichloride, 1,1,1-trichloroethane, carbon tetrachloride, 1 ,1,2-trichloroethene, 1,1,2-trichloroethane and tetrachloroethene. When the catalytic combustion was underway, exiting gases from the top of the reactor were analyzed in a chromatograph, except for hydrogen chloride, which was analyzed by titration with NaOH. The following data were obtained from these three experiments:

The outgoing gas stream could be used in this

form into an oxyhydrochlorination reaction.

The present invention thus provides a

new and improved method for handling unwanted chlorinated by-products which are normally obtained from the production of chlorinated ethylene derivatives, for example the production of vinyl chloride. The catalytic oxidation according to the present invention also includes recovery of chlorine found in the waste products,

in the form of hydrogen chloride, which can then be used in the oxyhydrochlorination step in the production of chlorinated ethylene derivatives.

Hitherto, hydrogen chloride has been extracted from them

unwanted chlorinated by-products during combustion using methane as fuel. However, this method is expensive and not reliable. Moreover, such a process is very impractical after

as the cost of recycling is more than five times higher than the market price for the hydrogen chloride. The present process, on the other hand, is economical as it does not require additional fuel, which significantly reduces recycling costs.

The present new method is also advantageous because

the temperatures used make it possible to extract heat for the production of steam or for preheating supplied streams in the production of chlorinated ethylene derivatives. A further advantage of the present process is the fact that essentially no elemental or free chlorine is formed, which causes only negligible corrosion of the equipment. Additional advantages of the present invention will be apparent to those skilled in the art.

Suitable embodiments have been described above

of the present process. Other modifications of this process within the framework of the invention will be obvious to those skilled in the art.

Claims (11)

1. Process for the production of chlorinated ethylene derivatives comprising an oxyhydrochlorination step whereby hydrogen chloride is reacted with oxygen and ethylene or a chlorinated ethylene derivative, characterized in that unwanted chlorinated ethylene derivatives and other by-products are separated from a product stream in the above process, that the separated stream is injected into a fluidized bed of a silicon dioxide combustion catalyst that is fluidized with air and kept at atmospheric pressure or superatmospheric pressure and a temperature of approx. 400-500°C, forming a mixture of hot combustion gases which essentially does not contain hydrogen chloride and is essentially free from both elemental chlorine and chlorinated hydrocarbons, and that the gas mixture is recycled to an oxyhydrochlorination step. 2. Method as stated in claim 1, characterized in that the pressure is atmospheric pressure. 3. Procedure as stated in claim 1. characterized in that the pressure is 0.17 - 1> 05 M? a overpressure. 4. Method as stated in claim 1, characterized in that the combustion catalyst has a surface area of at least 175 m /g« 5. Method as stated in claim 1, characterized in that the chlorinated ethylene derivative water is vinyl chloride. 6. Method as specified in claim 1, characterized in that heat energy generated in the fluidized bed is used to preheat incoming streams to the process for the production of chlorinated ethylene derivatives. 7. Method as stated in claim 1, characterized in that the pressure is 0.17 - 0.69 MPa overpressure and that the combustion catalyst has a surface area of at least 175 m <2> /g. 8. Method as stated in claim 7> characterized in that the chlorinated ethylene derivative water is vinyl chloride. 9. Method as stated in claim 6, characterized in that the pressure is 0.17 - 0*69 ^a overpressure. 10. Method as stated in claim 9> characterized in that the chlorinated ethylene derivative water is vinyl chloride. 11. Method as stated in claim 6, characterized in that the chlorinated ethylene derivative is vinyl chloride, the pressure is 0.52 MPa overpressure and the temperature 450°C"